The annual cycle of growth and rest is a fundamental adaptation for plants in temperate and boreal climates, allowing them to survive periods of environmental stress, such as winter cold or drought. This state of suspended development is known as dormancy, a highly regulated physiological process. It acts as a protective mechanism, ensuring that tender new growth does not emerge prematurely during a brief, false warm spell in the middle of winter. Plants must satisfy a series of internal and external requirements before they can safely resume active growth.
The Internal Requirement: Chilling Hours
Before a plant can respond to spring warmth, it must satisfy a deep, internal requirement called the chilling requirement. This phase is known as endodormancy or true dormancy, a state where the bud will not grow even if exposed to ideal conditions. This mechanism prevents premature bud break during unseasonably warm periods, protecting the plant from subsequent lethal frosts.
The chilling requirement is quantified by “chilling hours,” representing the cumulative time a plant’s buds are exposed to temperatures typically between 32°F and 45°F (0°C and 7°C). Temperatures outside this range are generally ineffective or can negate the chilling effect. The accumulation of these hours works like a physiological clock, resetting the plant’s internal growth inhibitors.
Meeting this requirement results in the breakdown of growth-inhibiting hormones, such as abscisic acid (ABA), which maintain deep dormancy. Once enough species-specific chilling hours have been logged, the plant’s physiological clock is reset. The plant then transitions out of endodormancy and becomes competent to grow, moving into the second phase, ecodormancy.
The External Signal: Day Length and Temperature
Upon exiting endodormancy, the plant enters ecodormancy, where growth is externally controlled by environmental factors. The plant waits for reliable external signals to ensure the growing season has truly arrived. These two primary signals are increasing day length, known as photoperiod, and sustained rising temperatures.
Photoperiod is the most reliable seasonal indicator because daylight length is constant year after year, regardless of temperature fluctuations. Plants use photoreceptors to measure light duration, confirming the days are long enough to support a full growing season. This signal prevents the plant from being fooled by a mid-winter heat wave.
Sustained temperature is the final, direct trigger that initiates the shift out of ecodormancy. This cue is tracked by scientists using “growing degree days,” which accumulate heat above a certain base temperature. Once the photoperiod is long enough and temperatures are consistently warm, the plant commits to awakening.
The Biological Process of Awakening
Once the chilling requirement is met and external signals confirm spring, the plant initiates metabolic changes marked by a shift in hormonal balance. Inhibitory hormones like ABA decrease, while growth-promoting hormones, particularly gibberellins (GAs), increase.
This hormonal shift activates dormant enzymes, resulting in a surge of metabolic activity. Stored starches accumulated in the roots and stems are broken down into simple sugars. These sugars dissolve into the vascular system, leading to sap flow.
The mobilized sugars and nutrients are transported to the buds, which begin to swell in a process called bud break. The emergence of new leaves or flowers signifies the full exit from dormancy and the resumption of the annual growth cycle. This commitment to growth is irreversible.
Factors Influencing Timing Variation
The precise timing of dormancy exit depends on genetic, geographical, and local environmental factors. Different species and cultivars have widely varying chilling requirements. For instance, some peach varieties may need only 400 chilling hours, while some apples require 1,500 hours to break dormancy.
Geographical location plays a role, as latitude influences both chilling hour accumulation and photoperiod sensitivity. Plants adapted to northern latitudes often have high chilling requirements to prevent premature growth during long, cold winters. Microclimates, such as urban heat islands or sun exposure on a south-facing slope, also locally affect the rate of temperature accumulation and bud break timing.
The reliability of these cues is challenged by climate change, which introduces uncertainty into the environmental signals. Warmer winters can result in insufficient chilling accumulation, leading to delayed, erratic, or reduced flowering. Conversely, if the chilling requirement is met early, an unseasonably warm February can trigger premature bud break, leaving vulnerable new growth susceptible to late spring frosts.